Democrats embraced superdelegates in 1982 to make sure party leaders have a say in who is nominated. By giving key insiders more voice, leaders hoped to avoid what some saw as a mistake in 1972, when George McGovern won the nomination but was a weak general election candidate. About 15 percent of the total delegates this year are superdelegates, and critics say this group has given Clinton an advantage. She holds a commanding lead over Sanders, however, even without counting the superdelegates.

Supporters of Sanders are especially incensed about the use of superdelegates, who can back any candidate at the convention regardless of how people voted in primaries and caucuses. Republicans have no equivalent to superdelegates.